Adding prebuilt_python_library to deps does not rebuild pex

[evanj]

When adding or removing prebuilt_python_library to the transitive deps of a python_binary, the pex does not get rebuilt. This causes me to have to rm -rf buck-out to get the correct output.

Steps to reproduce:

1. Check out the example bug repository: git clone https://github.com/evanj/buckbug
2. Build the binary that is missing a dependency: buck run //somebin:

[-] PROCESSING BUCK FILES...FINISHED 0.1s [100%] 🐳  New buck daemon
[-] DOWNLOADING... (0.00 B/S AVG, TOTAL: 0.00 B, 0 Artifacts)
[-] BUILDING...FINISHED 1.7s [100%] (3/3 JOBS, 3 UPDATED, 1 [33.3%] CACHE MISS)
Traceback (most recent call last):
    ... TRUNCATED ...
ImportError: No module named pycparser

3. Edit the python_library rule in somebin/BUCK to depend on //third_party:pycparser
4. Re-run buck run //somebin (which should re-build the pex and execute it correctly):

[-] PROCESSING BUCK FILES...FINISHED 0.1s [100%] 🐳  New buck daemon
[-] DOWNLOADING... (0.00 B/S AVG, TOTAL: 0.00 B, 0 Artifacts)
[-] BUILDING...FINISHED 1.3s [100%] (4/4 JOBS, 1 UPDATED, 0 [0.0%] CACHE MISS)
Traceback (most recent call last):
    ... TRUNCATED ...
ImportError: No module named pycparser

5. If you inspect the pex with unzip -l or run buck run --verbose 10 you will see the pex has not been rebuilt.
6. Clear the build state: rm -rf buck-out
7. Re-run the target: buck run //somebin

[-] DOWNLOADING... (0.00 B/S AVG, TOTAL: 0.00 B, 0 Artifacts)
[-] BUILDING...FINISHED 1.9s [100%] (4/4 JOBS, 4 UPDATED, 1 [25.0%] CACHE MISS)
pycparser: <module 'pycparser' from '/Users/ej/.pex/install/pycparser-2.17-py2.py3-none-any.whl.23f5bf5c584e3052492053082449b44eb9e5053e/pycparser-2.17-py2.py3-none-any.whl/pycparser/__init__.pyc'>

(this is the correct output)

8. Remove the rule from deps and re-run: It will still work, indicating that removing the deps doesn't force a rebuild either.

If you follow these steps with a python_library dependency, it works as expected.

Buck version: 8ad7e8dfe1c6af5e608608ddef677836652dda0a System: Mac OS X 10.11.6

[Coneko]

cc @andrewjcg

[c4urself]

Got the same issue: buck version v2017.10.01.01